Asbestos exposure increases paracellular transport of fibrin degradation products across human airway epithelium.

The inflammatory response to asbestos fiber inhalation suggests that the distal respiratory epithelium is an important early target of asbestos-induced injury. We have previously found that asbestos exposure increases the fibrinolytic activity and mannitol permeability of human airway epithelial cell monolayers. Because fibrin degradation products (FDP) are potent inflammatory mediators, we asked whether asbestos fiber exposure would increase the transepithelial flux of FDP into the interstitial space. To stimulate the pericellular environment following fiber deposition, asbestos-exposed epithelial monolayers grown on permeable filters were covered with human plasma containing fluorescein isothiocyanate (FITC)-labeled human fibrinogen. After 24 h, nearly twice as much FITC-FDP appeared in the abluminal chamber of asbestos-exposed monolayers compared with unexposed controls. This did not result solely from increased degradation product production because asbestos-exposed epithelium was more permeable at all apical FDP concentrations. The proteins that crossed asbestos-exposed monolayers included biologically relevant high-molecular-weight FDP, as demonstrated by streptavidin blotting of biotin-labeled FDP. We also found that FDP flux was not vectorial, was not saturable, did not involve proteolytic processing of FDP, and did not require active transport. Thus asbestos exposure increases the paracellular flux of intact FDP across human airway epithelium. This represents a novel mechanism whereby fiber-induced epithelial dysfunction may initiate and sustain inflammation in the distal airspace.

Asbestos exposure increases human bronchial epithelial cell fibrinolytic activity.

Chronic exposure to asbestos fibers results in fibrotic lung disease. The distal pulmonary epithelium is an early target of asbestos-mediated injury. Local plasmin activity may be important in modulating endoluminal inflammatory responses in the lung. We studied the effects of asbestos exposure on cell-mediated plasma clot lysis as a marker of pericellular plasminogen activation. Exposing human bronchial epithelial (HBE) cells to 100 micrograms/ml of asbestos fibers for 24 h resulted in increased plasma clot lysis. Fibrinolytic activity was augmented in a dose-dependent fashion, was not due to secreted protease, and occurred only when there was direct contact between the plasma clot and the epithelial monolayer. Further analysis showed that asbestos exposure increased HBE cell-associated urokinase-type plasminogen activator (uPA) activity in a time-dependent manner. The increased cell-associated PA activity could be removed by acid washing. The increase in PA activity following asbestos exposure required new protein synthesis because it was abrogated by treatment with either cycloheximide or actinomycin D. Therefore, asbestos exposure increases epithelial-mediated fibrinolysis by augmenting expression of uPA activity at the cell surface by mechanisms that require new RNA and protein synthesis. These observations suggest a novel mechanism whereby exposure of the distal epithelium to inhaled particulates may result in a chronic inflammatory response that culminates in the development of fibrotic lung disease.

Effects of calcium channel blockers on calcium uptake in rat aortic valve allografts.

BACKGROUND: The life span of human aortic valve allografts is finite, and many fail because of cusp rupture or calcification. Subcellular changes occurring in aortic valves in response to transplantation include the uptake of calcium. This study uses a heterotropic rat aortic valve transplant model to determine whether the calcium channel blockers diltiazem and verapamil might attenuate leaflet calcification. METHODS AND RESULTS: The 60 rats studied were divided into the following groups: 1) control: valves from normal, unoperated F1 generation of Lewis and Brown Norway cross (LBNF1) rats; 2) control: valves from syngeneic transplant combinations (Lewis/Lewis); 3) control: valves from allogeneic transplant combinations (LBNF1/Lewis, donor/recipient); 4) experimental: valves from allogeneic strain combinations treated with 30 mg/kg per day diltiazem; 5) experimental: valves from allogeneic strain combinations treated with 30 mg/kg per day verapamil. Drugs or saline (controls) were administered with osmotic pumps placed subcutaneously 2 days before transplantation. Animals were killed 3 weeks later, and the valves were harvested and prepared for calcium analysis. Energy-dispersive x-ray microanalysis was used to measure the calcium in a section of one leaflet from each valve studied. Paired t tests showed that allograft valves treated with diltiazem or verapamil contained significantly less calcium than allograft controls treated with saline (p < 0.001). When all five groups were subjected to one-way ANOVA, the valves in the allograft control group contained significantly more calcium than all other groups. All other groups were not different from each other. CONCLUSIONS: The calcium channel blockers verapamil and diltiazem were effective in preventing early calcification that occurs in aortic valves after transplantation. Thus, these agents might play a role in prolonging the life of human aortic valve allografts.